The present invention relates to a printing mechanism for printing bar codes on pressure-sensitive labels adhering to a carrier tape comprising a print wheel including a plurality of type rings rotatable relatively to each other about a common axis and a pressure roller for pressing the carrier tape with the pressure-sensitive labels adhered thereto against the print wheel.
Such a printing mechanism is already known from DE-PS No. 2,333,155. This known printing mechanism comprises a print wheel which is made up of a plurality of type rings whose periphery has the form of a polygon with ten equally sized straight side faces. On the these straight side faces types formed from metal project each in the form of a bar code element. The bar code elements disposed on the ten straight side faces correspond to the numbers 0 to 9. Since the print wheel is made up of a plurality of type wheels the type rings lying adjacent each other on planar side faces can represent multi-digit numbers in bar code.
In the known printing mechanism a carbon paper tape and a carrier tape with pressure-sensitive labels adhering thereto is led past the side faces of the type rings disposed in a printing position. For pressing the carrier tape with the pressure-sensitive labels and on the carbon paper ribbon against the bar code types on the print wheel a pressure roller is provided which is mounted rotatably about an axis extending perpendicular to the axis of the type rings. For carrying out a printing operation said axis is moved transversely of the print wheel and perpendicularly to the axial direction so that the pressure roller periphery rolls along the surface of the print wheel formed by the planar side faces of the type rings and thereby presses the carrier tape with the pressure-sensitive labels and the carbon paper ribbon against the bar code types. In this rolling operation the carbon paper ribbon produces imprints of the bar code types on the pressure-sensitive label.
As is known, pressure-sensitive labels with bar code types are read with the aid of electrooptical reading devices. However, to enable these reading apparatuses to read the bar code without errors close tolerances must be observed as regards the width and the spacing of the bar code types. A high contrast between printed and unprinted areas on the pressure-sensitive label is also necessary for correct reading.
With the known printing mechanism outlined at the beginning the conditions for satisfactory reading with the aid of an electrooptical reading device can be fulfilled only by overcoming great difficulties. These difficulties start with the manufacture of the type rings themselves because their side faces with the bar code types must be planar to an extremely precise degree. Furthermore, the bar code types of adjacent type rings disposed in the printing position must lie exactly in a plane so that in the printing operation carried out with rolling of the pressure roller on said plane an imprint of the ink from the carbon paper ribbon onto the pressure-sensitive label which is as uniform as possible is obtained. Even if the surfaces of the bar code types lie in only slightly different planes irregular imprints are produced on the pressure-sensitive label and consequently satisfactory reading with the aid of the electrooptical reading device is no longer possible.
The axis of the pressure roller must be guided during its displacement movement transversely of the print wheel with exact spacing from said wheel to obtain a uniform printing of the bar code types over the entire width of the label to be imprinted. Different application pressure intensities produce irregular imprints which lead to erroneous readings. Irregularities in the level of the bar code types and also the guiding of the pressure roller axis could be compensated by using a soft pressure roller surface but it has been found that this results in a decrease in the contrast obtainable on the pressure-sensitive label between the areas which are imprinted and those which are not imprinted. In particular, it is found that when the pressure-sensitive label is pressed from the carrier tape back by means of a soft pressure roller the microroughness present in the surface of the pressure-sensitive label cannot be pressed smooth and consequently surface pores remain uninked. This can be regarded as the cause of defective contrast when using a soft pressure roller.
The objective of the present invention is to provide a pressure mechanism of the type outlined at the beginning such that without excessive expenditure uniform and contrast-rich, i.e. reliably readable, bar codes can be printed.
According to the present invention this is achieved in that the type rings have a circular cylindrical peripheral surface at which bar code types of resilient material are formed, that an inking means is in contact with the bar code types to be printed and that the pressure roller is rotatable about an axis which is parallel to the axis of the type rings and consists of non-resilient material. The type rings from which the print wheel of the printing mechanism according to the invention are made up can, because of their circular cylindrical peripheral surface, be made with simple means to exact dimensions. The use of resilient material for the bar code types permits excellent transfer of the ink taken up by the inking means to the pressure-sensitive labels and the pressure exerted by the pressure roller of non-resilient material may be made so large that by means of the resilient bar code types the ink even penetrates into fine pores of the surface of the pressure-sensitive label. Consequently, even wide bar code types give uniform ink imprints on the pressure-sensitive labels which can be read with great reliability by means of electrooptical reading devices.